Fluid process systems typically use valves such as, for example, rotary valves to control the flow of process fluids. In general, rotary valves typically include a flow rate control member disposed in a fluid path and rotatably coupled to the body of the rotary valve via a shaft. Typically, a portion of the shaft extending from the rotary valve is operatively coupled to an actuator (e.g., a pneumatic actuator, an electric actuator, a hydraulic actuator, etc.), which operates the flow control member. To couple the actuator to the valve shaft, a lever or lever arm is typically employed. The lever converts a rectilinear displacement of an actuator stem into a rotational displacement of the valve shaft. Thus, rotation of the lever causes the valve shaft and the flow control member (e.g., a disk, a ball, etc.) to rotate to increase or restrict the flow of fluid through the valve. In operation, a positioner may be used to control the displacement of the actuator to rotate the lever and the valve shaft and, thus, the flow control member of the valve to a desired angular position to achieve a desired fluid flow through the rotary valve.
Industry standards (e.g., International Organization for Standardization) provide a means to couple an actuator to differently sized valve shafts. Adherence to the ISO standard requires that actuators and valves made by multiple or different manufacturers can be interchangeably coupled to each other without requiring modification of the actuators or the valves. To facilitate the compatibility of control valves with a variety of actuators, many available actuators have shaft couplings such as, for example, a square bore that can receive differently sized square valve shafts.
Some known collets have a cylindrically-shaped outer surface or portion having a substantially circular transverse cross-section that engages a cylindrically-shaped inner surface of the lever. However, cylindrically-shaped collets having engagement ends with substantially circular cross-sections may be prone to lost motion between the collet and the collet holder or lever if a torque reversal occurs. During throttling applications, the actuator imparts a torque to the valve shaft via the lever to cause the flow control member to change rotational directions between a clockwise and counter-clockwise direction to achieve a desired flow rate through the valve. This change in rotational direction may cause a cylindrically-shaped collet to slip at the connection between the collet and the lever, resulting in lost motion between the collet and the lever. In general, such lost motion may lead to inaccurate positioning of the flow control member and, thus, poor control of the flow flowing through the valve.